Shot peening is a type of cold working, and makes shots, which are metal or non-metal balls, impact on a metal surface at a high speed to generate a compressive stress on the metal surface to improve fatigue strength thereof against repeated loads. For example, to improve fatigue strength of a welded joint in a pressure vessel of a chemical plant or a reactor vessel, the shot peening is treated thereto. Patent Document 1 discloses an ultrasonic shot peeing apparatus that shot-peens a J-weld between a bottom surface of a reactor vessel head and a nozzle stub, and surfaces near the J-weld.    [Patent Document 1] Japanese Patent Application Laid-open No. 2006-346775 (0015, FIG. 2, FIG. 3)